This invention relates to a new chemical process for making esters of carboxylic acids. The process includes the preparation of commercially useful polyester plastics.
The esterification of alcohols with carboxylic acids is typically carried out in the presence of acidic catalysts. This reaction is reversible and a major problem is finding a method of displacing the reaction equilibrium so as to obtain a high conversion to the ester. This is often accomplished by removing water and/or ester by distillation as the reaction proceeds.
Acid chlorides and acid anhydrides react more readily with alcohols than the carboxylic acids themselves and require much milder reaction conditions. These reactions are irreversible, but they produce coproduct acids which must be removed from the reaction mixture or neutralized.
Transesterification, the reaction of the ester of one alcohol with a second alcohol to make the ester of the second alcohol plus the free first alcohol by an ester interchange or alcoholysis reaction is well-known. This reaction can usually be run with either acidic or basic catalysis.
Organic carbonates, the diesters of carbonic acid, can be prepared by reactions analogous to those listed above for carboxylic acids generally. A commonly used procedure for making carbonates is the reaction of an alcohol with phosgene, the acid chloride of carbonic acid, in the presence of an acid acceptor. Polycarbonates are produced in this way, e.g., the condensation of bisphenol A with phosgene in the presence of pyridine.
All of these conventional esterification methods involve various disadvantages including the use of acidic or basic catalysts, the production of acidic coproducts, and the use of an acid acceptor as well as the need for special product separation and purification procedures associated with such methods.
It is known that cyclic carbonates react with carboxylic acid halides to produce corresponding haloalkyl esters of the carboxylic acid with the elimination of carbon dioxide (see VanGijzen, U.S. Pat. No. 3,256,305). The process is limited to the reaction of certain cyclic carbonates and the production of a particular class of substituted esters.